Sonar systems have been widely used on marine vessels—e.g., surface ships, submarines, and torpedoes—for various underwater purposes, such as defining distances between objects, ocean floor mapping, and making other observations. In such systems, sonar equipment—like a sonar transducer or other form of hydrophone—can be embodied in or mounted on a hull of such a vessel. A streamlined housing referred to as an “acoustic window” or “sonar dome” encloses the equipment and protects or shields it from a body of free or open water surrounding it—such as an ocean, a lake, or water in a tank. The window is typically convex with respect to the body of water and embodied in the vessel to form a part of an exterior surface of the vessel and be contiguous and continuous with other parts of the vessel exterior surface surrounding the window. In this way, the vessel exterior surface is smooth and the acoustic window does not appreciably increase the drag.
An exterior surface of the window is in contact with the open water, and the interior surface is also in contact with water that is in a flooded chamber surrounding the sonar equipment. The window acts as a hydrodynamic fairing over the equipment and has water pressure on each side of the window. The window shields the sonar equipment from the moving water on the exterior of the vessel, this helps avoid noise interference that would be generated from the flow and/or cavitation of the flowing water around the equipment, and helps avoid vibration of the equipment as the water pushes loads into it. Typically the water pressure and force on each side of the window is equal and in balance, except for the hydrodynamic forces created by water movement due to vessel maneuvering. One requirement of the acoustic window is that it must withstand these hydrodynamic forces without significant deformation.
Desired acoustic-waveform energy (or sound-wave energy) is usually sent as signals from a transmitter located within the housing defined by the window, passed through the window to an object located without the housing, and reflected back from the object through the window to a receiver also located within the housing. As such, the signals propagate through the window in both directions. Another requirement of the acoustic window is that it should be sufficiently “transparent” to these acoustic signals, meaning it should transmit the targeted frequencies at the necessary range of incidence angles with minimal/acceptable signal distortion or attenuation.
It can be difficult to optimize both these structural and acoustic requirements in the same acoustic window design, and often there must be a trade-off of one against the other.
The acoustic window has traditionally been constructed as a single rigid sheet of high-strength materials—e.g., metal (such as steel) and/or fiber-reinforced plastics. However, the rigid window can generate and transmit a significant amount of acoustic noise associated with flow of water over the window and arising from vibrational frequencies related to operation of machinery aboard the vessel in which the window is embodied. The rigid window can also affect or generate a significant reflection of the signals impinging upon the exterior and/or interior surfaces of the window. Such reflection can result in a substantial reduction in the intensity of the signals being transmitted through the window. And, when such reflection occurs from the interior surface of the window during attempted transmission of the signals from within the chamber, spurious or erroneous determinations and/or echoes can result.
Other acoustic window designs have been utilized which improved upon the basic single rigid sheet configuration. For example, U.S. Pat. Nos. 4,997,705 and 6,831,876 each disclose a window made from a sandwich structure including a core layer sandwiched between and bonded to two septa (skin) layers. The material for and thickness of each of the core and septa layers are selected such that the window meets the structural and acoustic functional requirements. For instance, the septa have been made from materials such as fiber-reinforced polymers and metals. The core has been composed of low-shear/high-elongation-to-break materials, such as natural and synthetic rubbers, elastomers, and castable filled and unfilled synthetic polymers. These designs have been able to meet the structural and acoustic requirements of many applications.
However, these acoustic window designs are subject to limitations and have not been found totally satisfactory for all possible applications. More specifically, in “lower frequency” applications (up to about 40 kHz), an optimal design can be found in which the core and septa layers are relatively thick, which is typically sufficient for structural needs. In “medium frequency” applications (about 40-100 kHz), the core and septa of designs that are acoustically optimal (or even just acceptable) tend to be fairly thin. This results in difficulty balancing acoustic and structural needs. In “high frequency” applications (over 100 kHz), even modest structural requirements can become difficult or impossible to meet with acceptable acoustic performance.
Sophisticated instruments have been developed that are configured to use efficiently transmitted signals of high frequency (over 100 kHz) to increase definition and accuracy. Thus, there is a need for an acoustic window that meets high frequency acoustic requirements and the typical structural requirements.